RF interference? through RCAs after adding immobiliser

[tim_s]

hi fellas,
Wired up a new immobiliser to my car this wkend. long story short, all is fine, but now through my front tweaters I'm getting a ticking noise once the car's fired up, probably in time with the injectors. I wired a ground from the immobiliser to the point by the fusebox, which is where i assume the head unit gets its ground from. tried hooking the ground from the car stereo straight to the -ve terminal of the battery suing a jumplead to rule out a ground loop problem but no change, so i'm guessing this is ok. if i disconnect the front RCAs from the head unit end, get much less interference - the right side is fine, left a little ticky. if i unwire the rcas from the amp (in the boot) i get no interference at all. Most usefully (and i think this is getting closer to the problem), if i disconnect the antenna from the head unit, i get no interference. sounds like it needs an aerial isolator or something, but don't know what's changed to make that the case?
what's going on?! I have cheap RCA leads so not well shielded, but i had no problems before, and my symptons don't suggest to me that i ought to change.
other weird thing i need to look at is that the head unit fired up when i had disconnected the ground lead to the head unit, with everything else connected. i thought this was well weird. if i then disconnected the aerial into the head unit, it went dead as it should be!
what's wrong? what's changed! It's looking really likely that its something to do with the aerial, but i'm not sure what. I'm going to unhook it properly tomorrow and take a look, i was a bit short on time today.

Any ideas much appreciated - so annoying that installing an immobilser can screw up my stereo!