Suspected related problems between head unit, alarm, A/C
 

I have a Mitsubishi FTO 1995
I pulled out my old head unit and replaced it with a different one which turned out to be worse, I then put the old one back in after using the 2nd one for a few weeks but now it wont work.

When I turn the ignition on till lights show the alarm starts going off and the A/C starts up, the head unit also does not turn on at all, I thought it might be a fuse but checked them all and they are all fine.

I also can use the remote to lock the car and turn on alarm anymore.

Any help would be appreciated.