running two stereos in car

Hi all

Looking to running two stereos (OEM unit connected to CD changer and an Alpine IDA X305 dedicated mp3 unit) in my 2001 Golf

I guess I would need a relay for each speaker connected to the amp power send of the second unit to switch the speakers over but what options would i have on powering the units themselves? Is it simply a case of running another wire from the battery for the second unit or can i use the one thats already there somehow? 

use a harness and connect both together, but leave the connection 12v connection untouched. Run a 10-12 gauge wire from the battery fused at 20 amps to the deck area, then run your constant 12v sources off of that.

that way you dont have to worry about turning both on and blowing the radio fuse.

just remember, use one or the other only.

two 4PDT relays to isolate/select the four channels of speakers, wire the relay coils in parallel connected to the remote out lead of the Alpine.  NO contacts of relays to the Alpine hu, NC contacts to the OEM hu, C contacts to the vehicle speakers.  may need to boost the output of the remote lead with a trigger module, depends on current output available on Alpine and relays used.  should be fine powering both headunits with stock wiring. Alpine unit has priority over OEM unit.

m