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