Low-voltage common ground Q?

I have replaced my factory head unit in a 2004 Audi (non-bose) with an aftermarket unit. for now (until I get more $$$), I am using the head unit speaker outs for my front speakers, which is how the factory head unit worked. I do, however, want to use the factory rear amp that powers the rear doors and small sub by using the head unit low-level pre-amp outs. This head unit supports doing that. My problem is this, however: while the fronts work great and are floating ground, the inputs for the factory rear amp are l/r common (-) ground. The amp DOES convert this common ground input to a floating ground output before sending to the rear speakers/sub. So, with that known, can I splice/combine together the l/r (-) low-level pre-amp outs of the new head unit to form a common ground that my factory rear amp will accept?

While most want to convert speaker level floating ground outputs to common ground via an adapter, I'm just trying to do it with low-level outs that will get converted back to floating ground within the amp.

It has been done by others by just splicing the l/r (-) pre-amp outs, but I want to know if this may eventually fry the head unit.

Thanks in advance!