I'm afraid that I have searched long and hard for 6 volt LED lights and have not found any. That is why I also run halogen bulbs in my tail lights. If you're not getting enough light from them you may need larger wire. I did. I figured to light up the two halogen brake light filaments together I needed 9 gauge wire so ran such from the starter solenoid through a circuit breaker and relay switched by my brake light switch (current for the halogens is too much for the brake light switch - burned out my $35.00 Ron Francis switch in two months.)
I'm not sure what you mean by the "bulb and halogen" combo you're running. I just put this combo tail light and brake light halogen bulb in my single socket and works fine (with the heavier wire explained above, of course.)
Available here:
http://macsautoparts.com/early-v8-fo...0R3CHL1113361/