Left tail light is out ,also the lic.plate light is out, I have replaced BOTH bulbs 2 times just in case 1 set was bad. What else con be the problem?

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Do the Bulbs have 1 or 2 center pins? Could be a ground, any corrosion around side of bulb socket? Can you swap bulbs with right side to test? Also be sure bulb was not installed 180 degrees out if that socket type. Don't know the year bulbs went from 1156(single filiment bulb), and the 1157(double filiment), to the newer style.
I would go to auto parts store and buy the $5.00 tester that has a bulb, a metal point to touch electrical connection with and a wire with alligator clip.
Then clip alligator clip to socket ground and touch tester to each of the socket pins, bulb will light if power is present, if it doesn't light connect to known good ground or chassis.
If you touch the side with probe while hot, a fuse will blow. Good Luck.

I have used a multimeter at the lamp sockets and the connector plug, no juice.

Try the 4 way flasher and see if the other pin pulses. Verify Meter is working by checking battery voltage.
Check the harness for a bad connection inside rear of vehicle.
The right tail, left tail light, and license plate light is 1 wire, if you can't locate the problem, you could run a new wire from right side to left side.
If right side stopped working, a fuse may have been blown by meter probes.
Did you measure from chassis ground, and then from bulb ground?

I did measure from ground. And bulb ground. Meter works fine.. I will try the new wire...

If possible follow wiring from 1 side to the other. I bet you may physically see the problem. That probe, or a meter with a needle will allow you to lightly puncture the wire
To make contact for testing, just follow from good side to bad side, Not sure about your caravan, but may be connections over center of both back doors.
Also follow tag light back ti the harness. Your problem is in the last 1 foot of you van.
Luckily it is not your Stop/turn light, then you would have to deal with wiring from front to rear of vehicle plus your signal head. Good Luck, You'll get it.