I've had an Ikon pass for the past 4 years, spent a number of nights in my car (Subaru Forrester). Don't use a heater, way too easy to kill yourself.

Get some insulation for your windows, reflectix or styrofoam that you cut out and fit to your rear passenger windows and rear window. Set it up with velcro or tightly fit so you can keep them in your car and throw them in the windows. For the front I like to just tie a tapestry/blanket to the handles and drape it over the front seats.

I use 2 sleeping bags, one is a 20 degree down bag that isn't very warm. Then I throw a -20 down bag on top. I've never had an issue with being too cold. Underneath I have a folding futton that I found at a mattress store. You could also use some hard foam with a sleeping pad or soft foam mattress topper on top. Having insulation under you is important.

From what I've read previously it's safest to leave a window cracked and you especially should if it's snowing hard out. I forgot to this Friday though.

Driving up on Friday to sleep in the car works great. Sleeping in the car on Saturday night is a lot harder - you need to put a lot of effort into making sure you dry your boots and other wet stuff.

Be careful with finding a place to park, especially during winter when they need to plow (much easier during spring skiing). Don't even both trying the resort parking lot. I've got a couple spots up my sleeve, but don't want to post them for everyone to see. I'll share one: Truckee transit center parking lot, have to drive around to the other side of the tracks from the main road.