Can you use something like a Y RCA cable to multiplex two inputs from like a HU and a Ipod (Some other device) to an amp? I am not sure if this would cause noise or if it is even recommended.

Would using a relay to switch between the two inputs be another solution?

 

You are somewhat misuseing the term "multiplex".

No, you shouldn't ever do that - the outputs often interfere with each other, even when not on.

Many switchers use relays or ICs to do this -Soundgate and PAC have them, and Peripheral. Check Autotoys.

 

I wouldn't recommend using relays. I soldered up one with RCA jacks and a transister gain to power the relay, and I found the contacts in the relay affected sound quality (even when the coil was not energized). I would buy something like Eld. recommends, rather than build one.