Is there an optional auxilary cooling fan availiable for a ML55 that plugs in to the existing wiring harness? Has anyone installed one?

Nope, the ML55 has it's own powerfull suction fan just like the
ML500. The factory left out the harness provisions for the aux
fans up front. When the 55's suction fan kicks on to 100%,
watch out... you might get sucked in. That thing is beefy and should be able to cool in even the worst situations.
DR.D