Weird speedometer/aftermarket stereo problem

[01SOMZ71]

2001 gmc sierra slt

-aftermarket kenwood stereo

 

I've had this pickup for about a couple months now, Ice had a problem start where my speedometer was off By 5 mph. It wasn't a big deal to me. I ignored it. After about a few days of it being off by 5mph, it had risen up to be 10 mph off, and the next day after that if was 20 mph off and still is to this day. Some days it will be off by 10 or even by 5. The aftermarket stereo has no fm/am radio signal,so I took the radio out and unplugged it to see if I needed an radio atenna adapter and I noticed while the stereo was disconnected from the harness the speedometer wasn't off and dropped back to 0 while parked. I went for a drive and the speedometer was fine. When I plug that stereo in it jumps back up to 20 again. All the wiring from the factory radio harness to the aftermarket stereo looks okay the colors match. I'm puzzled on why it does this. Also the speedometer kinda wiggles a bit when turning power on to the pickup  like as if a surge is happening. Any help or advice of what to do would be great!

Edited February 6, 2019 by 01SOMZ71

[riverbanks]

Thinking, maybe the radio has a internal flaw?what if you,say,call crutchfield,ask about issue, maybe wrong harness adapter?

[greatmizzou]

The original factory radio had some sort speed/volume control setting.  Like the faster you go, the louder the volume gets.