Everytime I start my engine I blow the instrument light fuse. Last week I installed a new tach and I think I screwed something up in the process. Here's what I did...I pulled the old aftermarket tach which was spliced into the dimmer switch with one of those cheap blue plastic splice joints. I cut this wire off leaving the blue plastic splice thing on there. I installed the new tach but have not yet connected it to the dimmer switch. Now I have some sort of gremlin that blows out that fuse everytime I start the engine, if I turn the lights on without starting the engine everything is fine but the second I turn that key it's sayonara fuse. Any ideas?

Thanks

Dimmer switch gets one hot wire when the headlight switch is on. Then it directs that to either high or low. Nothing to do with ignition voltage. You definitely got something crossed up to ground that shouldn't be.
Try hooking the light in the tach to the gray dash light circuit when you hook it up so that it works with the rest of the dash lights instead of being full bright all the time.