A quick search of the Google shows several wireless options. I remember this Indiegogo campaign from last year. What is your budget? Please post up what you finally decide on.
https://www.indiegogo.com/projects/the-wireless-solar-backup-camera-that-never-dies#/
Submit a DIY to the forum;
http://986forum.com/forums/boxster-general-discussions/70654-submit-diy-article-receive-10%25-off-your-next-order.html