1984 300SD

this morning while i was trying to start the car i cranked it a few times (not sure if the battery went dead). after i took the key back to the no.1 position to try again the car won't turn over.

i checked all the fuses and they were all good. when i turn the key to the cranking position the lights dim in the interior. yet there is no attempt to turn over no matter what.

i thought the battery was dead but the radio, and interior light still works. and i thought if i am low on battery the starter will start clicking.

what can i look for to corner this problem? is there a relay for the starter i could have blown? or could i have fried the starter? any anvices will be appreciated

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198? 300D ???K - Future DD
1989 300TE 285K - traded in '12
1984 300SD 186K - Sold in 08
1978 300SD 330K - Died in 05