Take a look at this option:
Airmar GH2183 GPS Heading Sensor 10Hz
Product ID: AM1101-000 MFG ID: GH2183-10
Airmar GH2183 10Hz combines GPS positioning and highly accurate heading information in one compact antenna. The GH2183 eliminates the need to install a GPS antenna above deck and a heading sensor below deck. Only one installation above the deck is required, saving installation time and money.
Pretty bloody expensive, >$700! Even the basic (GPS only) is >$200.
Still leaning towards the Garmin (~$150), as I've since discovered that my primary problem isn't my old steam-driven GPS, but the driver for the serial/USB interface to my PC. I just upgraded from Vista (shit) to Windows 7 (OK so far). My old serial/USB cable doesn't have a 7 driver so I had to get a new cable but its driver causes NMEA communication to slow to a crawl (apparently a common problem). There's another driver which is supposed to work OK; I'll try it out tomorrow.....
I'll use the Garmin as a primary and my old one as back-up. At least the PC doesn't think the GPS is a mouse anymore!
Ain't PC/instrument communications fun!!