I know someone makes this, but simply, I am looking for a device that I can connect two separate preamp outputs into one input. Some sort of combiner, isolator thing-a-ma-gig. 12 volt powered.
You mean stereo to mono or to another type of single stereo connector?
Actually, one mono output, (GPS), one stereo output, (Bluetooth adapter, Bluetooth in, hard wire out) going into one stereo input.
https://www.autocom-shop.co.uk/
used to sell some of this stuff.
Found it!
The guy looking found and obsolete J&M accessory that will work.