Where to start looking....

The essentials:
1967 Barracuda FB, 273, floor shift A/T, Factory AC. Electric fuel pump, w/relay, & Battery Tender, are installed at the battery.
The story:
Every week, regardless of weather, I start, and let warm up my car. Gives me a chance to spend some quality time in the garage, with a couple of essential items of course, Beers and a cigar!
3 weeks ago (on Saturday), I installed my new tail light bezels. No issues at all.
2 weeks ago, I notice the Battery Tender "light" is Red. Battery posts had some corrosion going on, so I cleaned up both battery posts, and their corresponding cable ends, applied some dielectric grease, re-assemble, and wa-la! I get the Green light from the Battery Tender again!
Thursday evening, doing my usual routine in garage, step on brake pedal and notice only LH brake is lit (my normal operating procedure). I do some minor checking, and find out that I've got some type of wiring issue, and Sunday would be a good day to try and trace it down.
Today, I went to start the car to pull it out of the garage, and Battery Tender is showing Red again. I un-plug the unit (normal operating procedure), and turn key to start car, and I get nothing. No electric fuel pump, no exterior lights, no T/S, no starter, nothing! I do have courtesy lights, and the LH Brake light, but it seems weak, that's it! All three of the fuseable links look good, and I don't see any burnt/fried wires.
I am looking for a direction to go in, what flow chart of checking I should do. I am by no means an electrician, but I can usually fumble my way through a problem. I do have a meter, and a continuity tester at hand.
Any help is greatly appreciated.
Thanks in advance!
Steve~