I haz aux mod, and I love it.
I followed the parts list on post #2 of this thread:
[NB] Aux input - I'm close to giving up - MX-5 Miata Forum
It got me 5 switches, 5 plugs, and 50 resistors, but the 8 and 16 pin connectors were worthless, much too small to fit onto the pins on the back of the radio. All this was something like $15 shipped about 2-2.5 weeks ago. If anyone wants me to ship a few resistors, a plug, and a switch, lemme know. It just needs a cassette deck plug.
So I stole a plug from the MSM dash harness and dremeled it down to fit. Works pretty good. I also stole an in-line fuse holder (not sure if necessary) from the alarm off the MSM.
Then I followed these instructions:
Mark's Hangout - Mark's Hangout Blog - Custom iPhone or iPod Install in '99-'01-Miata
But mainly just this, cause he goes way too deep with the charging and docking station.
Then I used this pic for the left/right/ground orientation, cause I didn't know. I feel like this was something Joe Perez knew before even exiting his mother's womb.
And soldered it all together to make the above pic of input and switch. Works like a charm.