So, I wasn't able to plug in my 1981 300d about a month ago (it was a cold night, got down to I think 15 degrees) and of course it wouldn't start the next morning. Long story short, it has been sitting outside for about a month now, still not plugged in, and now I need to get it started. Would just plugging it back in at this point do the trick? I would have to buy a bunch of extension cords to do this, so I would rather not even try if someone with more experience tells me this will be futile. Its really cold right now, getting down to the single digits and I have no garage.
Thanks so much for any advice, I really appreciate it.

 

Well a bunch of Extension Cords is always handy to have; you could view it as an investment.

However, you did not say if your Glow Plugs were working OK?

If your Glow Plugs are working when the Glow Plug Light goes out continue to glow until you here the Relay turn off and you can try to start.
You can also do the above but not start; cycle your key and use your Glow Plugs again.
I have read of somey cycling their Glow Plugs 3 times to start on cold days.

Removing the Battery from your Car and keeping it in a warm place until you are ready to start the Car would also help. (by now it should need to be re-charged as the Clock draws off some current from the Battery.)
When the Battery is cold the out put is lower; if the Engine Cranking speed is low it may also not start.

 

Diesels need to be warm. Yes a little heat is the best thing . If you cant get power for the block heater where it is parked, maybe a small generator of 500 watts or over might help you. You can drain the engine of cooling water and warm it up in the house and put in back in the engine. Also good to bring the battery in to get it warm also.
Some people in cold areas of Canada, use a propane stove on low under the oil pan for about an hour. good idea is to put a flat cookie sheet over the burners.