Grab the starter relay from an old Ford truck.

From what little I remember, p = i^2 r = v i. 600 Watts / 12V = 50 amps so your fuse experiment is right. Better get a 60 amp fuse in case the alternator can’t keep up on a hot day.

Bulk quality wire is available from any MB with the battery in the trunk or under the rear seat. The W140 has a variety of lengths with terminals from the distribution center in the passenger footwell.

Sixto
98 E320s sedan and wagon
02 C320 wagon