Folks,

If the headlights had a short, or are drawing an excessive amount of current, you would be blowing fuses.......assuming that you have the proper fuses in place......

the following is for an 85 300D

Fuse #7......8 amp, white.........Headlight (RH High Beam)
Fuse #9......8 amp, white.........Headlight (LH High Beam) and Indicator
Fuse #11....16 amp, red...........Headlights (RH Low Beam and Fog Ligts)
Fuse #13....8 amp, white..........Headlight (LH Low Beam)

copied exactly as written in MB W123 Electrical Troubleshooting Manual

So, If you didn't connect those lights up directly without any fuse......then you should be having some fuse problems......if you have some sort of short!!

If it were me, I would get a voltmeter and check my Alternator's output...there are several threads relating to this......

I would also check my ground straps.....

I assume that you gave the parts store your old battery, in lieu of a core deposit......but I would have fully charged the battery, slowly, overnite....then tested it with my trusty Snap-On Resistance type Battery Load tester......

Luck to ya.....especially if you have custom improved on the factory MB headlight power distribution system.....

SB

__________________

Diesels:
'85 300D, "Max, Blue Benz", 155K, 27.0 MPG
'84 190D 2.2, "Eva, Brown Benz", 142K, 40.2 MPG
'77 240D (parts car)
'67 Eicher ES 202 Tractor "Otto" (2cyl, Air Cooled, 30HP)
Gassers:
'94 Ford F-150, "Henry", 170K (300 Six) 17.5 MPG
'85 190E 2.3, 148K....Parts Car
'58 Dodge W300M Powerwagon (Flat Fenders) Less than 10 MPG