One marten living under the hood isn't a problem, except possibly for the nesting materials and maybe BO, but when he leaves, and you park somewhere else and ANOTHER marten comes in, he destroys anything that smells like the other guy (very territorial little guys, these martens...). There go the wires, belts, hoses, anything their teeth are tougher than.
MB has installed "anti-marten protection" in the W126 and later classes -- switched off when you open the hood (that's what the switch that "does nothing" is). Pops a high voltage spark across the engine block every minute or so to tingle the feet of the little buggers and induce them to leave. I have no idea how well it works, doesn't stop mice at my house, all the cars get mouse nest under the hood once in a while.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!