Just a few notes from my install / experience following this DIY.
First, thank you wermbang. This was a very cost-effective mod that really makes-over the night-time driving experience. I would never have thought of doing this without your excellent, detailed post and pics. I know this thread is getting old, but I stumbled on it as one of the first results in a quest to find an LED bulb to replace the O/D / Overdrive Off light. (Its a T1.5-X: PC74 LED bulb by the way if a search engine steers someone else here).
I initially followed the first post instructions and bulb recommendations and found a lot of hot-spots in the end-result. Additionally, the process of modifying the twist-locks was a bit hit or miss and time consuming. I tried adding a dab of solder to the modified twist locks, but the finished product with all lights lit still didn't look quite right and it took a lot of tweaking and pulling of the gauge cluster to get it right. To assist any others and hopefully helps someone avoid spending time and cash on the wrong bulbs, here are my notes and bulb purchases:
All bulbs were purchased from superbrightleds.
Map Lights: I used two of these bulbs to replace the map lights in my 99. The blue is a lot dimmer than the stock white incandescent. I originally ordered seven of them when trying the twist lock procedure, but they had too many hot spots in the gauge cluster application. The dimmer and less blinding color temperature is easier on my night vision than the stock whites and seems like a great fit for the map-lights.
2 WLED-x5: 194 LED Bulb - 5 LED Wide Angle Wedge Base WLED-B5: Blue
Light-up key ring: Lighting isn't perfectly uniform around the ring, but it's not bad at all, looks slick to me and slightly brighter than stock.
1 74-xHP3: 74 LED Bulb - 3 SMD LED Wide Angle Wedge Base 74-BHP3: Blue
Bulbs for dash gauge cluster back-lighting: (comes with twist locks, no manual procedure necessary to retrofit, insert into the twist lock, then insert into the gauge cluster and twist). Mine went about a quarter turn past the original twist locks. Order seven if you want to switch out the turn signal bulbs. Lighting is very even, no hot-spots with the WLED-xHP5 compared to the WLED-x5 (several hot spots).
5 WLED-xHP5-PC: 194 LED Bulb - 5 SMD LED Wedge Base With Socket WLED-BHP5-PC: Blue
Ash Tray, Cupholder, Autometer Z-series: The 194 bulbs should fit the ash tray and cup holder lights. I pulled the color filter in the cup holder light. These are barely noticeable accent lights in the stock application. Could probably find something brighter / more directional for these, but they work without modification. I bought 5 of these total (but you only need 2 if your stock) to do the ash-tray, cup-holder and my 3 Autometer Z-series gauges. The Autometer gauges are lit almost perfectly by these lights and the gauge needles seem to fluoresce a little making it FAR easier to read at night.
2 WLED-x-x: 194 LED Bulb - 1 LED Wedge Base WLED-B-120: Blue 120 Degree $1.39
Climate Control Cluster: These bulbs should fit the Climate Control cluster without modification. Again, no twist lock gutting or repair. They were a little looser than the stock twist locks, but appear to be holding in place without modification. I did not add any additional LEDs to the CC lights and while the climate control cluster is slightly dimmer than the dash gauge cluster, I don't think it is that much dimmer that I care all that much. Perhaps OP's foil mod would help here (I skipped that step).
2 NEOx-xHP: NEOx LED Bulb - High Power Instrument Panel LED NEO3-BHP: 3mm NEO3-xHP Blue
O/D Off light / Other indicator lights: OP used this bulb for his high beam indicator. Rather than switch the already blue light on my rig, I used it for the O/D light instead. (I ride with O/D off 90% of the time). The blue light is nicer to look at than the stock yellow bulb, and now I can mentally note that yellow=warning lights, don't have to see one constantly on). I don't recall needing to modify the twist lock at all on this one.
1 T1.5-x: PC74 LED Bulb - Instrument Panel LED T1.5-B: Blue $1.79
I recommend AGAINST purchasing these for the climate control lights (instead purchase what is called out above in my post for the climate control). I ordered these based on the original thread,but I could not separate the bulb from the twist lock. I broke one in the process. I am not sure if the design changed, but these appear to have resistors in the base that are glued in. I could not use or re-use these for anything on the truck. If I get bored enough, I might skip the auto-parts store LED lights and try and use this in the headlight switch with a bit of glue and solder.
3 NEOx-x: NEOx LED Bulb - Instrument Panel LED NEO5-B: 5mm NEO5-x Blue
I also recommend AGAINST purchasing this bulb. I believe the superbrightled web-site suggested this for the key ring light. It wasn't on mine, and the bayonet mount doesn't have any application that I can see on a 99 2500 CTD. Not sure what to do with this one, or if I just ordered it mistakenly. If something tells you to use it on one of the automated parts finder sites, I'd suggest you check what's in there currently and save a couple dollars.
BA9s-x-xV: BA9s LED Bulb - 1 LED BA9SF-B-12V: Blue 180 Degree 12VAC/DC
Good luck all.
99 RAM 2500 CC 5.9L CTD