My check engine light has come on recently, but when hooked up to the reader, it only reads "ERR." It is not giving a diagnostic code. Everything else is fine. Has this ever happened to anyone else out there?
That could be the issue right there. If you matched the wiring harness colors exactly, including the amp power, then you've got power running back through the OBD... which would error out your reader (hopefully it didn't 'fry' it).
There's a few posts in the 'How to' section about this. (I'm not sure how to link there with the iPhone app).
This snippet from the site he linked to is really what I was saying in a nutshell.
"The problem is, VW brought the "K-Line", the wire on which all of the control modules in the car communicate with the scan-tool to a pin in the connector on which older radios used to put +12 back into the harness. (We're not sure whether this was for antenna power or as an amplifier turn-on or what). So if you were to take the non-diagnostics capable radio out of a 1997 GTI and put it in a 1998 (where there should be a diagnostics-capable one), it fits perfectly, plugs right into the harness, but the K-line ends up shorted to +12!"
The "K-Line" is getting power (+12) back through it. The dealership I went to to have my car checked was hesitant to plug their reader into the OBD due to this.
Anyway, since you had it installed by Best Buy, and they really should know this issue exists by now, I'd print off that website info and bring the car back in. Should be a simple fix, but if you do it yourself you will probably void their install warranty.