Unless you have a low current DC clamp on, you'll need to get in series with this circuit to measure current draw.
I guess if you knew the resistance of the element, you could estimate based on the voltage drop. I'm guessing it's something like 10 or 20 watts, which is a pretty small soldering iron, but a good amount of 14V current. What a waste to run the heater after the engine is in closed loop! There should be plenty of heat coming from the exhaust.
Most of the cheapo clamp meters are AC only, although they try to hide this in the sales literature by supporting DC amperage measurements through the leads, so be careful. To get DC amp readings in a clamp, you need a Hall Effect sensor in the clamp. I bought, and like my UNI-T 210D.
https://meters.uni-trend.com/product/ut210-series/#SpecificationsThis meter does all the usual stuff, plus DC clamp amps up to 200, for testing starters, plus capacitance, frequency and temperature readings via a supplied type J thermocouple. Best of all, it fits in the palm of my hand, and runs off two AA cells. 9V batteries in gadgets always seem to be dead when it's time to use the gadget, but you can always find some AAs around the house. UNI-Trend is a chinese company, but they have a long track record in test equipment, stay in their lane, value their reputation, and produce good quality, robust products, in my opinion.