..."its harder to heat the inside of a panel"
Well, it will also be harder to COOL it to. Ned, after lots of consideration, and looking the cost to fix the heater cores/valves ect....and then have to put A/C in it later, I have just about convinced myself to put the Heat & A/C system in mine that fits under the dash. If the A/C has a hard time cooling then I guess the same logic applies to it as the heater...hang a curtain up in the Summer same as the Winter.
You can sink a good bit of money in the regular heater system,money that could go toward the hole system.
Not for everyoneI know but another option you may want to consider. I really like the system that fits the contour under the dash.....just my nickel on this.
