You can tap into the wire at the firewall connectors, but so far I have been unable to locate it there - I cut it above the crossmember as that was the easiest area to access the harness. Please refer to Banks Trans-Command installation manual here:
http://assets.bankspower.com/manuals/188/96370.pdf
Figure 2 on page 4 shows you the location of the plug at the end of the transmission control harness, and figure 3 on page 5 shows the pin numbers of the plug - pin #4 is the one for the TCC lockup, that's the one the purple/yellow wire terminates at. The Trans-Command unit does not alter that part of the harness connector, it seems to me it only taps into the main 12V power and the EPC control wires, so it essentially piggybacks to the factory PCM and only alters line pressure through the EPC solenoid.
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'90 F350 XLT Lariat, reg cab, dually, RWD, 4.10s, factory tow package, and lots of add-on goodies
