I have a 2003 2500 5.9l auto 4x4. My issue is with the brake, and and air bag lights coming on and speedometer going to 0. Also loses the blower fan and air conditioning . All things are linked together and are intermittent. If I turn my blower control off, the lights go out and the speedometer comes back. I've replaced the vehicle speed sensor on the transmission, also checked fuses and relays for everything that seems related. I've also pulled the connectors on the and controller and they were squeaky clean. I would appreciate any help. Thanks, I've found numerous threads related to my problems but nothing quite the same.

 

You may want to look at the ignition switch. When the ignition switch on my 2003 failed it caused funny things to happen with the blower motor including staying on with the key off. Something to consider.