started getting harder to start, would have to let glow longer and longer, then one day it wouldnt start, after trying for some time the batteryseemed to go down, so i hooked up the jumper cables to my truck, the mercedes started up pretty quick with the cables attached. ran and started good for a week and now no start period, cables or no, i plugged in the block heater for a couple days - no start, not really coughing or sputtering when cranking maybe a tiny bit just for a second. no smoke that ive noticed while cranking.

Have you tried changing the fuel filters? Take off the fuel tank cap and listen for a rush of air which could mean a plugged tank vent.

Pump the primer handle a bunch (Like 50+ times) and see if it makes it any easier to start.

While it does sound like symptoms of glow plug issues, he did plug in the block heater. {providing it works.....}
That makes it sound more like the weak battery, slow starer scenario.......
I agree about upgrading the glow plugs, as that needs to happen whether it's the problem or not.

__________________
Jimmy L.
'05 Acura TL 6MT
2001 ML430 My Spare

Gone:
'95 E300 188K "Batmobile" Texas Unfriendly Black
'85 300TD 235K "The Wagon" Texas Friendly White
'80 240D 154K "China" Scar engine installed
'81 300TD 240K "Smash"
'80 240D 230K "The Squash"
'81 240D 293K"Scar" Rear ended harder than Elton John