The DRP (Distributor Reference Pulse) signal is the same between the TPI and TBI systems. DRP can be tapped by placing the scope input on the REF line from the distributor and ground on the engine block. What you'll want to view is the injector signal generated when cyl select is at TPI and TBI and that signal is likely to vary between firing all injectors with every DRP or firing alternating injectors with every DRP. There were a few tidbits on thirdgen and some of Ludis' schematics show the signals involved. I can try and narrow it down tonight if I have time.