The O-ring suggestion is definitely the first place to check. Also, see that you and your mechanic are clear with one another as to whether it's the shift cable or the park cable that's leaking; there is an O-ring on each, and the park cable housing also has a gasket.
If the cable jacket itself is damaged (as for instance if it fell down out of its floor pan clip and rested on the hot exhaust pipe, burning a hole in the plastic cable jacket, or if it cracked) then a repair is possible: You cut/sand away any jagged burnt parts of the plastic cable jacket, clean the cable thoroughly with brake cleaner, cut a piece of fuel injection hose (marked SAE J30R9) 3 inches longer than the damaged portion of the jacket, slit the hose lengthwise (some "spiral" to the slit helps), coat the damaged area of the cable with Mopar RTV silicone (don't substitute another brand), place the slit hose over the area and twist/shift it a little to spread the RTV, then apply hose clamps at 1½" intervals, just enough to hold the hose firmly, but not so tightly as to squeeze the cable, which would cause it to bind.
If in the end this doesn't work, replacement cables are available.
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