I recently installed a set of 4 x 6 MBQuart speakers in the front doors, and 5 1/4" in the back of my 73 mustang.

All the speakers give me sound, accept the front right. I know it isn't because I biased all of the sound to it and my car falls silent. I checked all the connections, short of popping the panel to check the actual "to the speaker" connection. Do you think I may have gotten a bum speaker?

easy to find out...take out the speaker and use a AA battery with two pieces of wire and brush them across the speaker terminalswith the other end of each wire on the positive and negative terminals of the battery...if it pops then it works....thats the first test...after that u can use an ohmeter to check some other stuff...check the speaker and get back to us....
Kevin

I guess, that means I have to pop the panel. Oh well, I don't have anytime now, so it might be a while before I get back to you guys.

no, you can also do this at the back of the head unit by disconnecting it and pop testing it with the right front speaker wires first. Then if theres no sound you will have to get to the speaker. That way you can eliminate the head unit.

Does it matter that I am running to an amp in the trunk?

Yes it matters. The fault could be anywhere,
the head unit, wires to the amp, that channel of the amp, wires to the speaker, or the speaker itself.

You have to systematically trace it down. In the vast majority of the cases, it's the wiring. Use the AA battery technique to trace down where the problem is.

Then you can start then by pop testing it at the trunk first. You can also just switch RCA's to see it the problem switches sides.