Arcturus;
6V Relays should be changed when going to 12V, as they will likely overheat with double V applied. See also:
https://www.sw-em.com/Ignition_Slave...us_duty_relays
...if you cannot locate a 12V Wiper Motor (first choice!), running the original 6V unit with a Dropping Resistor is always an option. I'm sure you've seen this, but here is the link again:
https://www.sw-em.com/Vintage_Volvo_...conversion.htm
...there is even some of your 12V converter info posted there, but this is for low current loads up to 2A...I expect these modules would not have an adequate output for the Wiper Motor...you could make some current measurements to verify (and remember: Motor current increases with mechanical load).
Cheers