Wire as a straight-through RS232 cable, pins 2, 3 and 5 are the only wires required. If your machine only has a DB-25 serial port (don't hook up the parallel port!), then use the pins listed for the computer-end connector, the MS-end connector should still have the DB-9 as described.

Thanks to Brian Dieckman for turning me on the the proper names for the parts of an audio jack. Here's a picture showing the TRS (Tip, Ring, Sleeve) parts.

 DB-9  DB-25  Phone 
23tip
32ring
57sleeve
(ground)

Phone jack is installed at some convenient location in the dash. Black glop is liquid electrical tape (easier than shrink tube, if you forget :).