those heaters work great. i've used them on many cars. i hook them up with a 12v relay - relay ground to chassis,relay power in direct to battery with 20A fuse,relay power out to the heaters and the relay control wire goes to the fuse box, connect to circuit #6 which is "on" when the key is at the glow light position. I then turn to the GP position with the key and let is sit to pre-heat the lines before starting, and adjust pre heat time depending on the outside temp. works great for me.
the other end of the heaters is ground, i connect it to the wing nut on top of the power steering pump.

__________________
1981 240D ... 265k
1977 240D ... 265k
1977 240D ... 250k