This is mickey mouse, but it works.
A neighbor has a Hummer (orig, not the H-2) the heater was n/g,
so he bought an auto tranny cooler (which is essentially a large heater core), ran the coolant pipes to it, put a valve inline to control the amount of heat, a simple blower fan to circulate the heat.

IT WORKS GREAT. looks like h*LL, but this will be a quick fix until you can afford tearing the dash apart. Find a place to put two 1/2 holes in the firewall, buy some heater hose and give it a try.

Me, I coughed up $500 (labor) and had a shop do it. Beg or borrow,
its not going to be a good winter without a heater/defroster.

If you run the revs high, the core can split like mine did, and
you will have an instant steam sauna inside, and I mean
clouds of steam, not drips!

In any case, Make sure you get all of the old coolant out of your carpet
I used a wet vac.

You could heat the car up with a 110v heater plugged into your house, then a small 12v heater might keep up.

Unfortunately, without heat, the windshield probably won't be your only problem, as the car will probably accumulate water vapor.

Have you priced the job at a garage? It might not be too bad compared to getting another car.
--
3 8s & 2 7s 725,000 miles total

Not likely. 120 Watts is about the heat output of an electric light bulb. Then you have to wire a ten-amp circuit to supply it. JC Whitney sells a little fan-driven heater that plugs into the lighter outlet, but its effectiveness in keeping a front windshield clear would be real questionable.