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Q: the modual behind radio {many mames} what all does it control on 1999 Mazda B2500

systems only appeared 3x a year at start and only in a very heavy downpour. where the wipers would not start and the transmission would not shift ti drive. if you pulled over and stopted and open the drivers door noticed doom light was not lit. fuse box incab has 36fuses. took out and reset ater checking each one good remember door open and dome lite not lit. i got to #25 fuse a 7.5 as i inserted back in the dome light lit op so i thought loose contact @ # 25 fuse started up and test drive shifted fine and speed meter working also this happened very rarely so i just woud reset #25 fuse. after a few months the rate of defect increased from 6x a year to 6x a month to 6x a day till presantly will not shift into high and speed meder not working . removed speed sesser on rear axcel tested it with a meter and magnet tested just like the new one but replaced it anyway to test truck. did not solve problem so replace with old again. the shifting with the speed meter must be solved . went looking i mean every where parts places and mazda for acompleat chassis wiring diagram to test for open circut have not been able to find . if you have acces please g.mail no wiring diagram or skymatic had shown the 36 fuse fuse block with fuse #25 or #28 shown this is the main system that must be solved....
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The module that has circuit protection in fuse 25 and 28 is the GEM module (Generic Electronic Module).

The GEM has a hand in doing many things - one of which is controlling interior lighting and knowing what doors are open and closed. Another is acquiring vehicle speed information from the VSS (vehicle speed sensor you replaced already), and providing that speed information to other components - such as speedometer and transmission.

The VSS and lighting feature ONLY have the GEM in this narrows your issue down even more so to the GEM.

When you pull fuse 25 and reinstall it, the GEM resets, or at least it used to.

Furthermore, when you initially had problems during rain, this is a clue also. Often times, you can get water intrusion from the outside that infultrates the GEM module. A wet module is never a good thing. You could have water intrusion also into your fuse box, which can cause the same issues.

My connecting the dots of your concern would tell me that you probably have a windshield seal leak (one replaced recently? Before the problem started at least?), allowing water to leak onto the GEM, which has gradually deteriorated the module.

Remove the GEM and inspect it for signs of moisture. If present, it will need replaced, but not before you fix whatever is causing the water to get in!

Let me know what you discover! Good luck.
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