That's what happens when I break my rule about dealing with swaps and unknown wiring. I should have asked if the flashing was rhythmic, rpm dependent, or just....flashing. Who knows what the light was connecting to. It may have been simply an intermittent short to ground as IJ suggested. If you're going to troubleshoot you'll need to disconnect it from the sender though.
Jon: You're welcome. Yeah, it's an goofy design. Think thermal telegraph. In hindsight I should have just scanned and posted the page out of the Body Electrical Manual. Its drawings, schematics, and text would have saved me all that typing. Here's a couple pics. This sender happens to be missing the resistor (goes across the prongs) used to cal the amount of current through the heater. The notched wheel with the ramp is a mechanical zero.