[Michael Yount]

And it's likely one of the existing 'penetration' points - the speedo cable/VSS or the backup light switch. Unless you've physically broken it (cracked the case, etc.) or recently disassembled/reassembled --- it's likely to be one of those two places. The backup light switch seals with a tapered thread; the speedo cable has a small o-ring seal. Is it possible that your longtubes have 'cooked' one of these bits? Check the o-ring on the speedo-cable/VSS. Mine 'weeps' a bit.

You can do what MFE suggested at home -- jack it up, liberally douche it with degreaser and hit it with the spray gun on a garden hose. That will clean it well enough for you to see where the leak is coming from once you drive it a bit.