Front dome light question
 

Greetings all...I have a 1977 300d & I am trying to replace a broken front dome light. I ordered the hella replacement and it has 3 prongs for the plug but my factory plug only has 2 slots & a ground wire on the other end of the light fixture. Asked my local MB shop & that was the only part listed for my year model. Should I just cut the connector and wire it up that way?
If so, what wire goes to what prong? Thanks in advance....