A few years ago I tried the SuperbrightLeds red 6volt LED (1157 replacements-maybe Bay15d) {looked like they had a cluster of plastic grapes on the facing end}.
They were OK, but they seemed to be the brightest if you were looking a them from the same elevation and directly behind them.
The best solution I found for getting attention of drivers behind me was to add more light fixtures and fit them with newer LED bulbs (wire them into brake light circuit). There are vast choices on fleabay and swap-meets of old chrome & painted bullet lights, beehive lens, clear fog, vintage tail lights, DO-Ray cab marker lights, winkey-cat, and old police/bus/fire/ambulance red lens lights to choose from. Simple"L" bracket can mount them to bumper or frame rail. etc. You can also modify the old internal light bulb socket to fit whatever 6V LED bulb mount you want.
LED's have very little current draw, and thus I haven't seen any dimming to other parts of the circuit. As a choice, LED in interior lights (like dome & glove box) are great in case they are accidentally left ON and they wont draw your 6volt battery down so fast as old incandescent. LED also don't melt plastic housings as bad as old incandescent.
Down side is that LED's don't usually mimic that old warm soft "glow" on dash (and old radio lights).
