Purple Starter Wire Blues

[AcampoDave]

I've been helping a friend diagnose a starter solenoid problem. It just barely clicks unless I jump it from the large battery cable post to the"S" post of the solenoid. The S post is still using the tiny factory purple wire from the ignition switch down to the nuetral safety switch, out the firewall bulkhead and down to the starter. The truck is converted to an automatic transmission and although the purple wire seems intact at all locations, it just doesn't seem to be delivering the power needed to kick the solenoid. The test light reveals that it functions properly and the light is bright when not connected to the starter, but illuminates more dimly when connected. I can hear a very faint click and the wire gets warm and these symptoms repeated themselves with a different solenoid. The starter works fine when I jump the posts and also when I test it with jumper cables on the floor. Is it possible that the factory purple wire just isn't heavy enough to handle the job of firing the automatic style starter. Also, a direct shot from the battery to the firewall side of the bulkhead will only kick the starter part of the time. Any thoughts?