Aboutbit headunit and the existing JBL system. I tried out many harness and wasted time/money/effort. I want you to make the job done quickly and economically. BUY METRA TYTO-01 for the Sienna 2006 with JBL amplifier system. Good Luck. Rear Camera included in this headunit has the RCA wire which is only 18 ft not long enough for a minivan.
https://www.amazon.com/Receiver-aboutBit-Bluetooth-Front-View-Subwoofer/dp/B07Y1VBYQ4