Check for shorts between the starter and battery---starter and junction post up on the firewall.
Check for a cigar lighter with some metal stuck in the recepticle.
For sport and amusement-----diconnect the harness at the bulkhead disconnect and see if it still happens.
This way you can narrow your search.
If it still happens----will be under the hood
if not---problem is in the cab or possibly and accessory lead to a trailer or winch.
I once bought a 78 4x4 with only 50k on it 'cause the owner could not find a short-----after a new starter and battery he gave up. $400.00
Turned out to be the battery lead to his trailer harness shorted and blew a fusable link. The wire burned inside the insulation and did not leave a mark on the link itself--------I DROVE the truck out of his driveway