You should be fine as long as there is a fuse in place. Size of the fuse really depends on what you plan on running on that curcuit. Keep in mind the circuit will always be active unless you install a relay that keys off the ignition key...

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280