Heating Coil for my '79 300TD Merc. Benz, heating c

It'd been cold here in the mornings now (in LA , no less) and my Merc Benz a few days go wouldn't turn over.  I called AAA to tow me to Sears (i thought i needed a new battery) but the AAA guy told me it was the Heating Coils.  I plan to tow to a mechanic tomorrow (Monday) and get it replaced but I would like to know how long can i expect a job like replacing a heating coil to take?  Is it difficult for a seasoned Benz mechanic or is it something that can be done in an hour or so?  I just don't want to get taken advantage of and want to call around with a bit of knowledge first.  Any help would be greatly appreciated very much, sir.

Answer
ive never heard of a heating coil, but if it won't even turn over its not that. with the key off turn your headlights on and see if the are bright or not. if not you need a bat. if they are turn the key on and see of the dim for a couple of seconds and then get bright again. if they do your glowplugs are working, if not you might need a new glowplug relay. after the lights are bright again turn the key and try to start the car. if the lights dim out then you need a bat. if they don't you will need to take it to the shop and let them check the electrical system to narrow it down.
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