I just found this
The FAST Harness (FAST-9999 and the FAST-BTAH) are not compatiable with your BTA10250D Tube, therefore you will not be able to use these with your 250D. The FAST Harness is only for use with the TA850 and BTAXX100 Tubes (as well as older model EL Series Tubes).
Purchase a amplifier installation kit to install your 250D. Be sure to include one that has a pair of RCA cables unless you already have a pair.
This is the answer i was looking for.
Sorry about the spam.
What other options do I have? The stock stereo doesn't have any RCA output.
PS I'm a noob when it comes to audio..