reconfiguring key on mac I type a lot Unix commands and I need often "~" and "`" which in U.S. keyboards are on the key under the escape key.

I'm now using an Apple spanish keyboard set to U.S. layout, and this key 
is mapped to the characters "§" and "±"  instead. I can't find 
"~" and "`" anywhere, and I need them exactly at their usual place anyway.

Is there an easy way to remap the key that doesn't affect other users?
(they use the Spanish layout).  I'd rather not use apps like Karabiner
that ask to "control my computer" as I don't know what else it does, if possible. Thanks.
I solve a similar problem using the free public domain utility Karabiner. This allows for extremely extensive remapping of single or multiple key presses (including control keys) to any specifiable Unicode character, plus other operations. It is frequently updated, and photos has an older version for much older versions of OSX. You'll need to play a little bit with some examples to get what you want, but there's also pretty helpful documentation.