Where to get a positive source on engine.

If you actually mean engine running—not power (IG) on or hybrid system READY, for which there are numerous sources—you might look at the Circuit Opening Relay Assembly (C/OPN) in the No. 2 Relay Block, which controls the fuel pump. In the Repair Manual (available by subscription to techinfo.toyota.com), see Engine / Hybrid System: 2ZR-FXE Engine Control: SFI System: Fuel Pump Control Circuit, and in the Electrical Wiring Diagram, see System Circuit: Engine Control, Fuel Lid Opener. Considering its importance for safe, reliable operation, however, I’d advise against any changes to this circuit.

If I were designing an accessory that had to detect when the engine was running, I’d check to see if this information might be available on CAN Bus 3, since it’s needed for the Energy Monitor display in the Eco app in the Navigation Receiver Assembly. CAN Bus 3 also already has an option connector (F62), and it’s separated by the Network Gateway ECU from more essential systems. The tachometer pulse (TAC) signal in the DLC3 connector could be another option, of course.

Without engineering design information Toyota doesn’t publish, it’s hard to say how any modification might affect safety or reliability. Indeed, just as Toyota does, I recommend against modifying your Prius at all, except perhaps by installing Genuine Toyota Accessories.