Where is the starter relay switch located on a 1999 Silverado 1500 4.8?
The starter relay on that 99 Silverado is located in the under hood power distribution center.
Does a 1990 Chevy Silverado have a starter relay you have replaced the starter and the switch located on the column under the dash and cannot find a problem with fuses or wires?
The starter relay is integrated into the starter. Have you checked the neutral lockout switch? If the neutral lockout switch is faulty or adjusted wrong the starter will never engage. It could also be that the starter wires are cross-connected. Using a volt meter, make sure you are getting voltage to the starter bendix when you attempt to start the engine. If not, start backtracking to the keyswitch to see where you DO have voltage.