Wire harness needed

Also, would a wire harness from a SLK 320 be the same as the one used in the SLK230???

try car-part.com , the search engine of salvage yards.

The SLK230 should be somewhat common in the salvage yards, due to higher production numbers.
You might double check wiring harness part number compatibility of your 2001 with the 2002-2004 refresh years. The 2001 is a transition year, so hopefully same as the 2002-2004. Avoid 320 or 32 cars, as the 6 cylinder cars surely have differences in wiring.

The space under the battery is a common mouse spot, we have had 5 R170s and 3 had evidence of rodents in this area when we bought the cars. Thankfully, didn't chew wires, but did mess up insulation and chewed a couple vacuum lines, had to fix those. Also found rodent evidence on drivers side under or near the fusebox and brake booster.

Happy top down spirited motoring, rodent free.

R