Power adapter polarity in vehicles

Shadowpanda

Seasoned Expediter
Hi all,
I am adapting my XM to use in my house. Rather than buy the 120v to 12v female lighter adapter I want to use alligator clips to connect to a supply I already had (for a HAM radio) using the male lighter adapter it came with. I cannot recall however on the lighter adapter whether the nub on the end is positive or the side push ins. I dont want to get this wrong in case the XM has no protection. Thanks to anyone that can tell me.
 
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