1969 Chevelle 396 A/T no A/C with gauges. Installed a NEW M&H dash harness, headlight harness & engine harness. Got everything to work,dash lights worked. Went to install front running lights,plug in rear tail lights & check to see if it all work. Dash lights went out. no running lights,no tail lights. HUMMMM..... Checked fuse...blown. replaced fuse..... turned on headlight switch,blew fuse. Double checked the items I had worked on. All seemed fine,no loose wires.Put in new fuse,turned on headlight switch,fuse blew. Tripple checked all conections. AS I mentioned. The dash lights worked perfectly for the last two weeks while building the car. I got the tester & checked the fuse box.I have NO power to the dash lights fuse on either side. WHAT HAPPENED????? ALSO,With the key OFF, I have power to the left side of the tail light fuse, shouldn't that be off with the key off? Remember,all the wiring harness's are NEW from M&H. NO burnt wires,nothing crimped,perfect condition. All I did today to the car is: install the front bumper,front marker lights,put new bulbs in the rear lights. NOTHING ELSE! Car starts,runs,drives still,tach works,but NO DASH lights,no tail lights,no running lights,no blinkers,no flashers.I even unpluged everything I did to see if the problem was still there & it still blew the fuse! What the hell do you think could be wrong????? Any help will be greatly appreciated!!!!!
Thanks!
Mike
msperformance @yahoo.com

................ALSO,With the key OFF, I have power to the left side of the tail light fuse, shouldn't that be off with the key off?.........

The key has nothing to do with the taillight fuse, it gets power when the headlight switch is pulled out with the ignition off or on.

Got to be a short somewhere in the taillight circuit.

took all the bulbs out of the rear harness,double checked the grounds.

Disconnect the rear harness by the fuse panel and see if the fuse blowing problems go away. If they do you can start working back from there looking for the short in the lighting wiring. If they still blow then you will have to look elsewhere.

You can leave out the fuse for the dash lights (the tiny fuse) and see if the taillights become functional. Again this can isolate the problem down to dash light wiring.

Then there is front light harness to examine for pinched wires.

I'll try that. The front harness is brand new.It worked fine the night before.The only thing I did was plug in the front runninig lights. I unplugged them & the the problem still persist.