Amplifying AUX input .. booster messing with RCA grounds

[grant2]

My car accepts AUX input via RCA plugs, for which I use a 3.5mm -> RCA cable.

The AUX output from my phone is weak (i.e., both phone + stereo volume must be very high to get decent volume) so I purchased a line booster: https://www.amazon.com/gp/product/B003LWV77Y/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

When I put the line booster between my phone + stereo, the signal is lost. Playing with the multimeter, it appears that the RCA output shields are being grounded to the car chassis. (the input RCA shields are, of course, connected to my device which the aux cable is plugged into)

I think i want the booster to not do anything with the voltage on the RCA shield (but amplify the voltage on the pin of the RCA cable)

The booster has these instructions which I suspect may be the answer:

We provide cut loops to facilitate noise isolation. To isolate, but both loops.
When the loops are intact, the output RCA shields are ground referenced: i.e., continuity to ground. Some power amps expect a differential input and can't deal with audio negatives trunked together... By cutting the cut-loops, you float the grounds so they are now "loosely" referenced to to ground through a capacitor and 100K resistor

My questions:

1. Am I on the right track in thinking I need to somehow get the RCA output shields to match the RCA input shields?

2. Does it look like cutting the cut-loops will do this?

3. If not, would a plausible solution be to cut the output RCA shields completely from the booster, and instead connect them directly to the RCA input shields?

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[Jeffdachef 5,000+ posts]

Try a fiio headphone amp instead or get a better head unit.

 

[grant2]

The stereo is stock and I won't be replacing it.

The booster I have is what i have, I picked it because it can be powered from the car and not require a battery like headphone amps. It could be replaced by something else I suppose but it'd be a lot more satisfying to understand and solve this puzzle!