DIY Mobile Line Output Converter

[MiniVanMan]

I'm looking for a DIY solution for a line output converter for a friend.

This is for a car, where the owner wants to use the stock head unit. I've been looking around at commercially available LOCs and they leave a lot to be desired, especially in low end output.

This is what I know. 6 volts, unbalanced speaker level outputs from the head unit. Will be going to a processor capable of line input levels of 250 mV to 7.5 volts. So, I have a lot of room to work with.

I guess the biggest issue is ground isolation. I could just buffer the speaker level with a resistor and see what I get, but I expect either a lot of noise, and/or turn on/off pops.

Not looking to spend a ton, and am getting kind of stuck on a decent, inexpensive transformer that will fit my needs for this situation.

Since I'm spending somebody else's money, I don't have the luxury of experimentation. Though learning something has some value for me so, I'm willing to shell a little bit out of pocket.