Is it possible to connect an existing OEM GPS antenna to the 8000NEX? I'd like to replace my 4.5" stock VW navigation unit with the 8000NEX, but I'd like to be able to use the existing GPS antenna.

Can't say for certain, but unlikely.  In short, it will require compatible connector/pinout and communication protocol.

FWIW, I installed my GPS module in the OEM location, behind the instrument panel and under the dash top (plastic, no metal above it).  Works fine and hidden from view.

A quick search brought me here:

http://www.amazon.com/gp/aw/d/B00DA35JTQ?pc_redir=1404658441&robot_redir=1

From this thread:

http://forums.vwvortex.com/showthread.php?6977265-Installing-Pioneer-AVIC-8000NEX-into-the-TTRS

I know guys on here, have used their oem antenna, but they were gm. Maybe someone will help with this.

A quick search brought me here:

http://www.amazon.com/gp/aw/d/B00DA35JTQ?pc_redir=1404658441&robot_redir=1

 

 

From this thread:

http://forums.vwvortex.com/showthread.php?6977265-Installing-Pioneer-AVIC-8000NEX-into-the-TTRS

Hmmm... don't know what the connector for the VW antenna looks like, but the other end definitely don't look likes it matches the 8000's...

Perhaps this one...

http://enfigcarstereo.com/ENFIG_GAA_PIO2.html

...from this page...

http://enfigcarstereo.com/vw_radio.html

Great info and helpful links! I'm looking forward to replacing the stock unit with the 8000NEX, while keeping the install as clean as possible. I'll update if I can get the OEM antenna to work. Now, bring on Carplay!

Try http://www.tss-radio.com/

I purchased an adapter for my 2011 Camaro to keep my stock XM antenna.

I also got all of my VW antenna adapters from Enfig. I have a 2013 Passat TDI SEL.

The GPS antenna adapter linked by jhren above worked perfectly for me. I also purchased an AM/FM antenna adapter and SiriusXM antenna adapter from them.