Is it the white line going to a vacuum motor on the heater box in the right corner of the engine compartment? This line is famous on these Fords for rotting out and leaking.

On my 95, I too wanted to keep it looking authentic. Ford doesn't sell the stuff, not even a complete harness. I got a chunk of 1/8" air line for a semi (hard plastic real similar to our vacuum lines) and spliced it in, even reused the original end at the vacuum motor. I'd like to find some in white and better hide my splice though. I just spliced it with a short piece of rubber vacuum hose.

To replace the whole line, you'd have to cut the tape in the harness loom and go in the cab and under the dash. I'd rather leave the the harness untouched.