If this is the same Rand McNally GPS I'm thinking of, it's black and shaped like a computer mouse. I bought this a while back for $90 and it works GREAT on the I-Opener. Included StreetFinder Deluxe software works fine as well.
It has a DB9 connector and a PS/2 plug to get +5V from the keyboard port. Since it only uses GND, TX, RX, and +5V, I connected those lines from the com port mod directly to the IO's (unused) modem jack, and stuck an RJ11 connector on the GPS. Snap the line in and go! (Had I used r_fl_z's RS232 daughterboard, it would have been even easier!)
Some local newspapers had a coupon for $15 off a $50 order at Office Depot, check yours and see. If not, maybe the cashier will simply accept the coupon code #4530. If so, you got a GPS for ~$40 :)