NINE.5 amp channel 1&2 not working

I have a NINE.5 car amp that is a five channel amp. Channel 1 and 2 are not working at all, the other channels work fine. Where is the best place to start?

Swap the input and outputs of channels 1 & 2 with some other channel and see if the problem follows it.

Thank you for the troubleshooting 101, got to make sure i am sane first!!! LOL. With the amp powered on (relays closed) I get a constant ohms reading on channels 3,4,&5. Channels 1&2 read wide ass open!!! Sounds simple enough, but this board is huge and complex. I am trying first to on-board troubleshoot the paths I can see. I'm also trying to avoid having to flip the board over as there are many soldered connections and heat sinks to deal with. No components are melted or open solder points. Any help on component/group component troubleshooting is appreciated.

What is the DC voltage (To ground) on the output of the channels that do work? And for channels 1&2?
Posting a picture of the PCB would also help us help you. One other thing you can do is look up the datasheets for the Audio amp IC's that it uses. They usually supply you with a typical application circuit which may be very similar to what you have.

Here is a pic. The back soldering doesnt look all that great.

Since you have 4 amplifier circuits, between the 2 relays which are basically carbon copies of one another, you could compare DC voltages between a working and non-working channel. You'd need to put it back on the heatsink for this. But, you could also do a basic in circuit diode check on the power transistors and compare a working to a non-working channel. Are you 100% sure that both relays activate and couple ALL amplifiers to the speakers?