The
46-50 service manual was comprised of 7 individual booklets and is in the literature archive but there was no electrical section issued as part of the manual -- just a wiring diagram which was published in a
service counselor. There is a decent electrical section in the 51-4 manual and some of the parts carried over and were used in later models. Even if the parts were not the same the instructions for service are for the most part applicable to your 50.
As to the temperature gauge, it is electric and uses a sender screwed into the rear drivers side of the head. One unfortunate thing about the 48-50 gauges is they were of different construction than all other year gauges Packard used. Most gauges used a changing resistance type sender but the 48-50 temperature and fuel senders consist of a contact on a bimetal strip which closes to ground so the gauges get intermittent bursts of 6v current flow. The senders are controlled by the action of a heater and external force of temperature or fuel level pushing against the bimetal strip. The external temperature (or fuel level) varies the pressure applied to the strip holding the contact so it takes a longer or shorter time for the heater to bend the strip to open and close the contact. The varying duration and frequency of the on/off pulses of 6v works another heater in the gauge which controls another bimetal strip that makes the needle move. The easily found resistance type senders of other years will not work properly with these gauges. You can see if the gauge functions at all by MOMENTARILY grounding the sender wire as you watch to see if the needle moves higher but without a properly operating sender you cannot know if it is reading correctly.
NOS Packard senders are getting scarce but some Ford products of that era used a similar system. BDeB posted that 36-55 Fords used a fuel sender that will work in Packards and I suspect the temperature senders will too but it will need to be the single terminal sender. Some Ford flathead V8s used two senders wired in series so one will have a single terminal and the other will have two terminals. What I don't know is if the threads that screw into the head will be the same or if some type adapter would be needed on a Packard. Ford vendor Dennis Carpenter sells repro senders.
The usual reason brake lights are not working is failure of the hydraulic switch mounted on the end of the master cylinder. Power comes from a circuit breaker located on the back of the instrument cluster and should be at one terminal of the brake switch at all times. From the brake switch in 23rd series and later models, the wire on the other brake switch terminal goes to the turn signal switch and then via two wires on to the brake light/turn signal bulbs. There could be a dirty turn signal switch or broken wire or disconnected plug elsewhere but in all probability the brake switch at the master cylinder is where to look first.
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