accessory power ports

Up front -The 2000 Suburban has a cig lighter port which has it's own fuse and two other accessory power prots

Then on the center console there is another power port and in the rear there is yet another one.

My power ports keep going out.  The one on the console goes out sep from the two up front.  The cig lighter is the only one working now and it has its own fuse which I replaced.

I have looked at every other fuse and cannot fund any bad ones.  There is an accessory relay but I cannot tell if it is bad or not. 

Does anyone know if there are fuses in the wiring sep from the fuse boxes?  Any other idea where I might look?  I use an inverter for various things and it is annoying that the power ports keep going out.  The dealer fixed them and just said that there was a build up of white stuff in them but I have looked and I don't see any build up.  In addition they act like a fuse has blown when they go out.

.