Its not for my bike but for my campervan! I have two head units in the van one in the cab which runs to a sub woofer under the seat the other which runs to the speakers in the rear. Can I wire the rear head unit to the sub as well  without causing damage to the front unit,  because the blue wire become  live to the sub from both ends, if  i were to wire there rear head unit up, if you see what i mean or would i need to put two switches one at each end of the two blue wires to Isolate one unit from the other? Any help would be appreciated Gary.                                                                                                                                                                             

Basically you have answered your own question. assuming that you will only be using one head unit at a time I would use a four pole double throw relay (maplin order code N42AW) to switch the audio for the sub woofer between the two head units and a double pole double throw (maplin order code N41AW) to switch the power from the respective head unit.    I would use the electric antenna wire from the front head unit to power the coil on the relays. That way when you turn on the power to the front head unit selects the cub other wise the rear head is selected. I'll scribble down a diagram and post it up here.                                                                                                                                                                               

I have uploaded a quick diagram off the top of my head. have a look at the tec album in my profile. It is an untested design but should work. use at your own risk. if in doubt consult an auto electrician or a car audio specialist. It is not a difficult thing to build just take your time can connect one head unit at a time.

Hi Paganprince Thanks for the response,  Clever bugger never thought about using the Ariel feed! I've asked a couple of car hi-fi dealers like uplands in Bristol and western car radio, but they are very vague about it and noncommittal, a auto electrician mate (who a fireman part time as its all villages around here  two miles Glastonbury ) said he was unsure as he would have to see wire diagram of head unit before committing himself. As both stereos are wired in already and independent of each other  one on the leisure battery and one on main battery,its the powering up of the sub thats the worry from either end and then sending 12 volts back up the feed line to the other stereo, could  use your idea and mount in next to the sub that way I could save on wiring (not cost just the routing of wires) And use one double throw switch? what's think. Gary. tried to print you diagram but it would not let me.                                                                                                                                                                                 

Managed to down load the diagram now and print it off! so off to maplins its a sixty mile round trip from here, still will be worth it if it works, and tips on burns? just in case I have to fight a loom fire. wish me luck, Saturday will be the day. will let you know. regards and thanks. Gary. Thought For The Day.   Treat every stressful situation like a dog, If you can't eat it or hump it, Piss on it and walk away.