I'll but in temporarily.
1) The dimmer is located here:
https://cgi.ebay.com/12v-12-v-LED-s...US-seller-/360276691806?pt=LH_DefaultDomain_0 Looks like a PWM controller for 12V DC
That out of the way. What are you using for a 12 V power source? Do you know the power requirements for the heaters?
I would reccomend playing with a single channel before wiring all eight.
This very well could be an undauting project for someone unfamiliar with temperature control and electronics in general.
The controller may or may not be suitable and it will be a wait and see prospect. I don;t have a feeling for on/off time, response time of the dimmer, the thermal mass or the temperature range being controlled.
It's definately a way different output model than I'm used to. I'll use the word inexpensive, but not guaranteed method of control.
You need a 12 V power supply. Current rating depends on what you need. 8 channels at 1A may be more expensive than it need be. That would be an 80 W power supply. Think of the heat output by two 40 W light bulbs. The dimmer is about the amount of heat from a 7W night light.
The "light" will effectively blink at some rate the dimmer in effect makes it a variable wattage light bulb.
All I'm trying to do is to get you to "visualize" the system.
Based on the info I've seen, your missing one key component. A 12 V power supply. Regulated would be preferred.
You need to have an idea of what the current and voltage requirements are.