soldering does provides a more reliable, more solid connection. BUT, and i empahisize "but", now that i think about it, i strongly urge against soldering because there will come a time when you'll want to disconnect the tail from the bike, for example, changing out sissy, and you won't be able to without cutting the solder joints. anyways for the job we're talking about, the perceived benefits of soldering won't really be realized. if we were talking about a higher current circuit like the charging system, then soldering would be beneficial. no, stick with the quick connects. i regret even bringing up talk of soldering, hehehe.
some guys report that they didnt get a resistor pack with their light, but i believe they were under strange circumstances..maybe eBay special, dunno. i got the resistor pack with mine and to the best if my recollection i can agree with you that the light-to-resistor connection was properly mated by the manufacturer. it's the resistor-to-bike connection that you'll have to customize.
Regards from my Taptalking Hercules Android