Ebay sells all kinds of clamp-on-to-the-mirror types for under $10. Just search "mirror mount GPS".
If your inside mirror is totally useless like ours was, but you don't have a backup camera monitor like we do here, maybe you could mount your GPS in this area. It might work great as long as you can quick-release it and willing to run an extra hidden power cable to it.
The hardware below is what I used for my backup camera monitor. It is found
HERE, Part Number: 250-8088, but it does not have a quick-release feature.