VW block heater. Diesels, particularly when they get high miles on them, may become hard starting in cold weather, the cheap solution for this is a block heater. This block heater taps in to your cooling system. When you plug this heater into an extension cord, it brings the engine up to "spring time" warm. If the engine will start in the late spring or fall, it will start in the winter with a block heater. It's a cheaper option than rebuilding the engine.
This 1000W block heater taps into your cooling system. Two key things to remember when installing it is to mount it as low in the cooling system as you can possibly go; this is usually below the oil cooler. Also make sure the flow to the heater is not going to be blocked by your thermostat.
Since you need to cut one of your coolant hoses to install this VW block heater, so you are going to need to top off your coolant. Rather than putting the dirty stuff back in, replace your coolant with new coolant that meets all VW requirements and make sure that you have full protection through the frigid winter.