Originally Posted by RedTitan20
Can I just buy a bypass from radioshack or some auto parts store?
If you buy a relay at Radio Shack for a DIY, look for the smallest one you can find with a 12 Volt coil. It only needs 2 switch contacts (closed when relay engaged).
The one on eBay is a huge 30 Amp automotive relay. Complete overkill for this application (and could even cause problems if you add a speaker amp -- the amp remote turn-on lead on some HUs may not be able to drive one of those huge relays plus multiple amps).
I recommend the smaller solid-state bypass units that you'll also see on eBay. They're only a couple $ more. Picture from not too long ago: