As the switch is completely unrelated to the kit and installation, there is no reason to think that it has anything to do with the malfunction.
However, several things come to mind:
1). Does the cassette deck work properly? Because if the deck is not working due to worn/broken belts and not in the reset position, it will constantly power on the unit regardless of what the power switch does. If this is the case, you will need to manually rotate the flywheel (in the proper rotation direction) to reset the deck which shuts it off.
2). When you cleaned the power switch, is there any chance you stretched one of the leafs? The power switch is an exposed switch that relies on calibrated spring leafs. A fulcrum action causes the leafs to spring one way or the other. Any distortion of the leaf springs will cause them to not work properly anymore. Rather than risk stretching or deforming them, the only way I ever try to clean them is to place a tiny piece of thin 400-600 grit sandpaper, folded over so grit is on both sides, and insert between the contacts. I then press contacts together and pull the paper out. It does not take much to restore the surface. Wetting the sandpaper beforehand with some deoxit can't hurt either. There are also 2 positions so make sure you got both positions.
3). Is the switch shaft lever actually installed correctly? If it came loose, you might not actually be switching anything.
4). Lastly, this may sound obvious and silly, but is the switch cap actually slipped over the switch handle? It is possible to miss and snap into the groove and miss the shaft.