Here are two posts from another thread i posted in. i hope you can gather what you need from here.

"After installing a new headunit everything in my car worked perfectly, about one month after the air conditioner would no longer come on. the screen on the stock hu still works and the little bars light up for the fan speed as well as the temp its at but no air. I took it to a local shop and they told me it was something to do with the aftermarket head unit.

yeah i still have the stock HU and an aftermarket one in the cubby hole. I've now taken it to both places and neither could figure it out. At the audio place he said he tested the compressors fuse and the amount of power going through the fuse was very faint. And the auto repair place said the compressor wasn't getting any power at all."

 

I'm going to take a wild guess that there is a ground backfeed.
12V pos + 12v pos = ZERO volts [potential difference]
I've seen this several times where a 'hot' wire is touching somewhere yet there is enough resistance to keep from blowing a fuse and the positive voltage backfeeds up a ground wire.
In any event I'd recommend you have a schematic [ETM] on hand and check to see if this circuit is controlled thru the multi-plex unit.

 

thanks for the response butt the honda dealership found it to be a bad transistor and its all solved now.