I've heard of these a few times on here, and I would like to use these on my Alero, so that I can make good use of my 10" and my 250 watt amp that divorced my Bonneville lol.
How do I go about using these..assuming I do know what they look like, my guess is, they are RCA out puts with inputs on the end that can be attached to the rear speaker inputs....also..my Alero has the factory Monsoon amplifier in the trunk on the left hand side tucked up in the fender..will this interfere much like the gain control on a Bonneville?

You have the right idea. Simpler line converters have speaker level inputs and RCA outputs so an amp can be connected to a source without RCA outs.

I once had a Denon converter that had RCA inputs too. This unit was more than just a line converter, it was allso a level controler. This allowed me to tweak the output levels to match different amps.

As far as appearance, they can look like a small circuit board covered in heat shrink tubing with wires coming out of one end and RCA connectors coming out the other. Or, like the Denon, it can look like a small amp without the fins and lots of inputs and outputs.

How well do these line convertors work? My amp has both line level and speakers level input. Would I be better off converting them or just running them into the high-level inputs? My deck has only speaker level outputs.

I got my converter at Best Buy for about $20, and it works great. Just to compare, I tried wiring my system first using low-level, and then high-level inputs. Low-level wins hands down....much cleaner sound.

I just upgraded recently but I was in the same situation. I was using my factory head unit so I got my Line converter from Best Buy about $20. It looks like a rectangle box has about 3 sets of dual wires coming out and then the 2 RCA holes. I wired 2 or 4 (can't remember) of the wires right in the area behind the head unit to the rear speak wires and ran rca'* from there back to the amp in the trunk and it worked great. I was running a 250x MTX 2 channel amp and 2 10" subs in the box enclosure. I have since changed my head unit to one w RCA preouts just as a preference.

I'm running a line converter, but I don't like it at all. I've noticed that it actually takes away from the sound. It'* like the amp is stealling the juice that'* supposed to go to the rear speakers. I never use it any more.