I know this topic has been covered before, but I thought I would provide some additional information from my experience.
I decided to convert the taillights on my ERA to improve my visibility to other vehicles. I have the round Lucas lights.
After doing a little research I purchased the 1.85" Radianz clusters with pigtails from Custom Dynamics
http://customdynamics.com . They are a little pricey, but if cost was the primary concern we probably would not be driving these plastic cars. They have a "direct" attachment bulb, but I did not think that the shaft of the bulb was going to be long enough to reach the base of the socket because of the protruding heads of the mounting bolts in my light housing (discussed and illustrated below).
Below is a picture of the LED cluster.
Typical installation instructions for these LED clusters suggest that you use Marine Goop to glue the clusters to the light housing. I did not take that approach.
In the picture below you see the heads of the two bolts that attach the light housing to the body. They are just a hair over 1¾" on center, so they prevent you from simply attaching the LED clusters to the light housing, unless you are willing to mount them off center.
I suppose you could layer enough Goop on the housing to fill the gap, but I also did not like the idea of a permanent installation. I chose to fashion a "mounting disc" that would be held in place by the bolts that attach the light housing to the body. The LED clusters come with adhesive tape on the backside and I planned on attaching the clusters to the "mounting discs".
I bought 2" cut washers with ¾" center holes and the notched them to fit between the mounting bolt heads. The thickness of the cut washers is approximately the same as the bolt heads. I then glued smaller washers to the cut washers so that the distance between the holes in the smaller washers was the same as the distance between the mounting bolts.
See the picture below.
I threaded the pigtails of the LED clusters through the big washers, inserted the pigtails into the light socket and then attached the washers to the taillight housing.
I then rotated the LED cluster clockwise in order to encourage the wire to curl up inside the pocket created by the socket and washer hole. I then attached the LED cluster to the big washer with the adhesive tape on the back of the cluster and Bingo, done!
The result of the installation is a secure but removable LED light cluster.
One concern I had was whether the LEDs would function properly when I used my turn signals. In some installations the low current draw of the LEDs make signals flashers think that a bulb is burned out, so they either flash too quickly or not at all. One fix is to install load equalizers in the circuit for each bulb. The downside of load equalizers is they become hot, so placement is critical. Fortunately the ERA flashers work fine so I did not have to resort to installing load equalizers.
Below are pictures of the running lights. On the right side are LEDs lights and on the left side are standard 1157 bulbs.
Standard Bulbs
LEDs
There is a noticeable difference in brightness, and from my observations side visibility does not suffer from the directional nature of the LEDs.