This chart shows some results for polyolefins, which are what most heat shrink is made of.
https://www.polyonedistribution.com/sites/default/files/0901b803808e059f.pdfNote: Different formulations differ in stability. I suspect one rarely knows exactly what a particular piece of made from.
1) Always stagger such soldered joints. I suspect you did that.
2) Heat shrink Teflon tubing is made, but you may not have access to it.
3) If you know what the wire insulation is, then that is probably safe to use is insulating tubing. We used to use carbon tetrachloride to soak tubing. That makes it swell. It will work on many different elastomers. The swelled tubing can then be applied over a joint and as the solvent evaporates, it shrinks back to its original size.
4) You can use a chemically resistant, non-heat shrinkable, thin wall tube, slide it over the junction, and secure it with a nylon tie strap or tied cord. Approved silicone tubing for fuel (it's usually clear yellow in the US) could be considered. It's not thin wall, but space does not seem to be an issue.
I suspect you have already got the job done. So, I would just see how it works. My usual solution for difficult to find automotive connectors is an auto junk yard. In fact, I just got the connector for the EVAP solenoid of my 2012 Ford Fusion that way.