Biba:
I'm confident that I could build/breadboard a circuit that uses the low current limit switches. It would be used in addition, to the relay set-up that your trying to build.
What I think can minimalistically work is a small switch such as
https://www.digikey.com/product-detail/en/c-k/T105SHZQE/CKN1088-ND (3A rating) and a Bi-color LED such as:
https://www.digikey.com/product-detail/en/everlight-electronics-co-ltd/MV5437/1080-1109-ND/ shown below.
Images courtesy of Digikey. There is a bushing that can be used to mount the LED.
The functionality would be as follows:
1) Low current limit switch operation: Around 10 mA
2) Transient protection/reverse polarity protection - nothing heroic
3) It should be powered with 12 V that need to go away if the motor fuse blew.
4) No undervoltage lockout
5) Flipping the switch on and off rapidly could cause the fuse to blow.
6) Pick a color for up - Green? (it's the safe position)
7) Pick a color for down - RED (It's an unsafe position)
8) So it would glow green when fully extended, and red when fully up. OFF when indetermanent.
9) No brightness control for the LED, although it can be fixed. Selectable with a component change. Probably not variable.
10) Breadboarded --> not a commercial PCB.
11) Screw terminals probably
12) No case.
13) It was mentioned somewhere that the spoiler could interfere with trunk and top operation. No provisions to stop that from happening.
14) I'd have the SMT parts soldered to a DIP header to breadboard the PCB.
15) I don't have a BOM (Bill of Materials) or time estimate yet).
It would be ideal to use some sort of connector and/or interface to the OEM connectors. e.g. the limit switches.
You would have to run a 5-wire (educated guess at this point) (shielded preferred) cable to the console switch area. Connectors are probably a sore spot,because the tool to crimp the wires is about $70.00. Delphi makes some nice automotive connectors. The LED can have a connector that connects direct. We would need to find a convienient place place to connectorize the dash area. You also need fused 12 V power for the spoiler. That same power has to power the circuit for safety reasons.
Possible options:
a) LEDs on the PCB that mimic the limit switches.
b) LED or LEDs on thr PCB that indicate applied polarity to motor.
c) A second switch/LED combo that can be plugged into the trunk connector.
Connectors
Mating connectors would be nice. Especially the motor and limit switches.
Notes:
Thermal PTC thermisters that get mounted to the motor have to be sized properly.
The fuse should be selected to blow if the motor hits a brick wall. To get a better selection, you may have to resort to another holder. Both of these have to protect against a failed limit or something gross getting in the way.
Future total unknowns:
a) How the switch is wired. We know there is a resistor inside and two terminals that select up down and one of these terminals probably goes to +5 via resistor.
b) The malfunction LED PROBABLY has two terminals? What is the voltage and/or current?
c) How is illumination of the switch handled?
The malfunction LED probably is probably wired directly to the BCM. I can't match the ICON to the BCM icon in the SM. The voltage/current is unknown.
So, the switch could have from 5 to 7 terminals on it? It needs to be reverse engineered to even think about using it. One issue that MAY need to be solved is the prevention of movement while the control circuits are powering up.
With significantly more effort, the OEM switch can likely be used, but now, I'll bet we don;t have access to colors.
So, you have options of "moving", "at any limit" and possibly "between positions". I think "between positions" would be the right indicator, but harder to do.
Illumination
I have no idea how the console illumination (dash lights) is done. Some radios use a "DIM" signal that just lowers the illumination brightness which is usually fed through say the parking lights being on.
Motor
Using the torque specs, it may draw less than 1 Amp. It would be useful to know what it draws no load, raising and lowering and even raising and lowering at high speed as well as stall. The DC resistance would be difficult for you to measure, but that would give me a ball park figure.
Knowing the current requirements for the motor can likely make the unit smaller.
Fully automatic operation
I'm not in a position to tackle this at all. With a microcontroller and a $300.00 USD interface it would seem you can get access to speed on the CAN bus in the engine compartment.
There appears to be 3 speed setpoints. deploy, retract and alarm (chime when not deployed when > some mph) "Chime" might add one or two more wires tot he front.
It would be somewhat useful to video the deployment/retraction and see if there is a program (VLC) that can view the file in slow motion, to figure out what the stepped actuation is all about. How many stages and how long they are.
Epilog
At this point in time, I might be able to do a PCB layout with an online type of program. It may change in the future. I definately have a large learning curve. I did it manually (no computer) some 40 years ago and with a DOS based program probably 10-15 years ago.
Does any one else want this thing? At what level of functionality? At what price points? I would want no liability. Besides that, I can't control how something would get installed.
When you rely on a fuse for protection, as soon as it's replaced with a higher value all protection goes out the window. That fuse is the only insurance against a failed limit switch. Hopefully a PTC attached to the motor can be added as well.