can I add music (mp3) files to an SD card that has a map data set on it

If you are asking about your nuvi 3590, it does not have a MP3 player, so there would be no way to play the files through the nuvi. Some of the older units (7xx and 8xx for example) had MP3 players, but none of the current models do.

As far as putting MP3 files on the SD card containing a map, it will not cause any problems in using the map. It's just that you won't be able to play the MP3 files through your nuvi.

Thant's why I still use my Nuvi 350 & 370, & they're still running great with the original batteries!

Fred

I do that on my 660... No problems having map & music on external SD drive.

Obviously your GPS needs to support music....

been doing that for a couple years now.

I'm not sure what Nuvi model you are referring to since your profile has the 3590 showing. That unit has 8gb internal memory so I don't know why you would have a map on an SD card.

As for the SD card, you can put any files you want on the SD card and it won't affect the ability for the Nuvi to use the Map.
The Nuvi will only use the files present that it's designed to read. It won’t read MP3 files but you can use the card in another device that reads MP3's.

You can even have movie files on it that can be played in a compatible device that is able to read those files, obviously the Nuvi will not.

By getting an SD card with 32gb (maximum Nuvi can read), or smaller, you can use it in several devices and still have the Nuvi read your Map.

In today's world when many cars have Bluetooth or usb accessible sound systems on board, why would you even want to listen to some crappy 1cm speakers even if your GPS was able to play them back???
My Nuvi 855 does play MP3 but I do NOT ever use that.