1999 Villager Estate. Brake lights fuse blows immediately....

Frustrated. 1999 Mercury Villager Estate. The brake light wouldn't illuminate. Found that the brake light fuse (20A) was blown. Put another in and it blew as I was inserting it. Figure there's a short somewhere, so I've disconnected both brake/tail light units in the back, the towing light connection, and have even completely disconnect the 3 brake light switches under the dash (yes, 3.... 2 directly attached to the brake pedal and one attached to the e-brake). Still, when I insert a 20A fuse (which is what is called for), it immediately blows. Since I've disconnected the brake light switches, it seems to me the short can't be past that on the way to the back of the vehicle. Maybe I'm wrong though. It seems the short must be either between the switches and the fuse box, or somewhere between the fuse box and the battery. Unsure if the wiring routes through any relays under the hood, but I'd think it would at least have to go through one of the relay boxes since that's what eventually gets directly connected to the battery. I've tried using my multimeter to measure the voltage and resistance on either side of the space where that fuse goes, but I'm unsure what to expect from a reading like that, so those results are inconclusive. Any ideas?

I've still been trying to troubleshoot this every evening after work. One thing I tried this morning was I went ahead and cut the yellow/black wire that goes towards the rear of the vehicle about 2 inches from the brake switch connector. The fuse still shorts as soon as I try to insert it. Pretty sure that tells me that the short must be in between the fuse box and the brake switch connector. I wonder if there's a relay in between those two points though? There's several relays right next to the fuses under the dash, but they're not labled anywhere, so I don't know what they're all for. I'm guessing the problem can't be in between the fuse box and the battery, else the short would simply burn the wire in two, right? This is being a real pain to figure out!