Ground Loop Isolator Makes It Worse

2004 Saturn VUE. For months I've had my Delphi SkyFi2 XM radio hooked to the aux input on my Blaupunkt head unit. When I installed it I noticed a loud engine whine. I installed a ground loop isolator, and the problem was solved. This weekend I went to install a small powered sub (using the RCA preamp outs on the head unit). It works great, but my XM receiver now does now. The sound was extremely muffled and sounded like robots talking in the next room. You can't even make out what song is playing. I took out my GLI, and the sound was back to normal, except for having the engine whine again. I bought another GLI (different brand, too), and it does the same thing. Other info: I had installed the original GLI backwards by mistake but didn't know it until now (input to output and vis versa). I unplug ONE channel of the RCA cables to the GLI, and sound is normal except mono in BOTH left and right speakers. If I plug in just one channel into the aux in without the GLI, I get only one side out. I'm not sure if this problem started exactly when I hooked up the amp or at some point during finishing the installation. So why is the GLI making the sound worse? Thanks ahead of time for your help.

I'll plug my ipod into the aux instead of the xm and see what happens. It's weird that the xm unit works fine without the GLI, just with alternator whine. But it craps out when I plug in the GLI. I might just take everything out and check all my connections again. What a pain!

Something is wrong with my aux input. I plugged in my ipod, and low and vocal frequencies are not heard. The unfiltered XM radio sounds fine except for the alternator whine. XM with the ground loop isolator only lets a small range of sound frequencies through, making it sound like garbage. I hope to have time to pull it apart after work today to see if there are any wiring issues.

I pulled everything and here's what I found: XM sounds normal with the car off. With the ignition on, I hear excessive whine. XM with the GLI sounds horrible, with only a few frequencies passing. Connecting only one of either channel is ok, but in mono. iPod connectly directly to the aux input has a wider range of tonal frequencies, but is missing low and vocal ranges. iPod with GLI is just as bad. If I connect only the left channel, it sounds ok but in mono. Both or just the right channel is bad. I determined that it must be a problem with the head unit. The battery powered iPod should sound fine no matter what ground problems I may have (keep in mind that the radio and CD player sounds perfect). So I'm taking advantage of the extended warranty I bought on the head unit and sending it back. Assuming that they will not repair it (they reserve the right to just refund my $100 purchase price in lieu of repair, and I can't imagine the warranty company having the capability or desire to repair it), I went ahead and ordered a new Kenwood head unit. It has a few additional nice features. There were some quirks to the Blaupunkt I didn't like. For instance, it would seem confused and not immediately respond to power on/off, sometimes forget there was an aux input and require that I restart it, no volume leveling of sources (my CD player would blast me out if I didn't turn it down before switching sources), and hard to use volume knob. The Kenwood also has a dedicated sub output. I'll report back when I get my new head unit. Hopefully this is the solution, and anyone else with this problem will gain some insight by searching and finding this post.