amisaacs

I know it was a previous topic but I couldn't find all of the details I was really looking for. I have a 2003 Silverado single cab long bed with a 4.8 frame that I am trying to put a 99 Silverado cab and bed on to. I tried to just swap the cab on and use the 2003 frame and motor harness with the 1999 cab harness plugged into the fuse box but I had plugs left over and when I tried to start it the gauge cluster wasn't working properly and it wouldn't turn over at all. Will I need to swap harness's and use the 1999 frame and motor harness for the whole frame or use the body harness from the 2003 into the 1999 cab? Any help and suggestions would be very appreciated