'89 Suburban timing

I just got an '89 Suburban C1500 w/ the 350TBI and am planning on checking the timing after I do a tune up this weekend. I have been told that I need to disconnect a wire from the computer or something before I can accurately check it. I have a '91 C2500 with this same motor, but have never had to fool with the timing and understand that the 'wire' is on the firewall by the relay box. Can anyone tell me where this wire is on my Burb and how to go about checking the timing?

If the emissions label is still there,It should asy on that.I do know the spec is 0 or 10 degrees.The wire is a black wire with a tan stripe

Spec is 0 with the wire disconnected (under the black power dist block cover).

With the wire connected, timing should be around 20 BTDC.

//2000CAYukon

Thank you all very much