I have 2008 nissan altima coupe. I have all the wiring done correctly just need to know if i could hook up the oem mic, or know how to splice it to work with the connection for a Pioneer AVH-X3500BHS DVD Receiver and where to patch into the reverse signal relay. A lot of the forums say to patch in to the reverse light, but is there a closer option and if so what would the color wire be?

Unless you have a backup camera the reverse gear input isn't needed at all for the head unit. If you're deadset on using the OEM bluetooth mic then you would have to splice into the wiring to it, in my experience its much easier to just run the mic that the head unit gives you to sit right next to the rearview mirror. It seems to give better quality sound to others, from you, on the phone imo.

I'm a newbie here, but I started learning about car audio since 4 years ago.

The Mic should be able to just find the wires and splice a mono headphone jack onto it to plug in (2.5mm if I remember pioneer right). The reverse light signal you should be able to get in one of the kick panels, but it would be easier to find at the tail light.