My son just picked up an 2003 3500 CDT with a 48re, it currently has about 62k miles on it. Of course there is a few issues with it, but will repaired as we go along. Now one of the issues he has and I don't know how we missed this when test driving the truck, he cannot lock out over drive. Hit the button dash light does not light, tranny does not down shift. Is there a relay(s) and fuses that I can check or what else could it be. any help would be greatly appreciated. No warranty 3rd owner.

 

The switch is probably bad.