hi bob,

get the excellent volvo service manual wiring diagram for your model year to
address wiring issues. it has everything you need including component
locations.
there's an additional relay for the e-style fogs called the auxiliary light relay. item #127 in the 1989 740 book. there's a spot for it in the main fuse/relay block.
i added an extra relay for ecode light conversion in a 93 940 that is similar to the canadian volvo circuit where they have drl. there's different circuits for different countries.

here's an 1988 740 book at ebay- you can look at the photos and see how detailed it is.
https://www.ebay.com/itm/164026450100?hash=item2630bd00b4:g:HYIAAOSwOhxeGOeb

search for the 1991 book. the wiring diagram files were all over the internet before, but now have mostly disappeared, replaced by what look like google proxy sites that claim to have everything, for a fee of course. the search engines don't readily cough up the free files anymore.

here's the engine circuits, only part of the 89 manual:
sorry the lights circuits aren't here.

http://www.autoelectric.ru/auto/volvo/740/1989/740-89.htm

maybe someone can point to a good link to the 91 manual?

good luck, bill