'99 4Runner MPX B fuse
 

Is the MPX B identified as MPX Multiplex Communication System? Mine keeps blowing out after my battery was drained and the altenator 20amp fuseable link blew. 20 amp link replaced, does not blow alt charges correctly. MPX B and interior radio fuse blown, both replaced, MPX B blows with engine off key out of ignition. Dont know where to go, only have Haynes 92076 Toyota book. additional info, pulled stock radio put in 15 year old raido, will drain battery if not turned off or pulled (removes from chassis, old school). think it might be harness pinch, but dont know where to start checking...

Opps!! my bad, not the MPX b fuse but the Dome fuse right benieth it......went through and found that it is the Antenna Relay (power antenna) that is "F"d up. unplugged it. no longer blows fuse......

Ha ha ha ha.

thanks for letting us know what it was.

Nice to have closure for you and others it may have happened to.

from YUonDAGround

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How did you know that it was the power antenna relay and where is that located at. I have the same problem with my Runner. Thank you.

YUonDAGround
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I used my trusty Radio Shack digital Multi-Meter (48 test unit) to go through all the items that ran through the Dome Fuse circuit. It took about 2 hours. Since I already suspected the radio unit I checked it first much earlier during my diagnosis. So I did not get back to it until I check everything else. It was fine on the second pass. But then I noticed on the wiring diagram (from the Haynes book) a reference to the power antenna. It is controlled by the relay WHICH IS LOCATED BELOW THE RADIO BOX, BEHIND THE CIGERETT LIGHTER. IT IS MOUNTED ON THE LEFT SIDE OF THE SUPPORT.