Hi Bob,
I drove my '97 Force all last winter, and found that the electric heater was really marginal. It provided okay warmth when the weather was 40 degrees F or higher, but I was pretty cold when it was below freezing.
I'm gonna recommend that you consider a kerosene heater. Solectria offered an optional kerosene heater, and used the Eberspacher brand. Ford, with their ThInk car also offered a kerosene heater and they used a Webasto.
The California CARB amended their zero emission mandate in the 90's to include kerosene as an allowable heating choice for EV's. They concluded that electric heaters were relatively inefficient, best used for mild temperatures.
Best regards,
Todd Martin