Osram DE3423 LED festoon lamp is one watt.  If you want ten seconds of
fading light at average half brightness that would require something like
five watt seconds of storage in your capacitor.  Say six or seven because
of the threshold voltage of the LED.  WS = C x V^2 / 2, so C = WS / (V^2 /
2).  So C = 7 ws / (12V^2 / 2) = 7 / 72 = ~0.1 farad or 100,000 uF.

Here's one that would serve.  It's nominal 40 mm diameter and 80 mm length,
and it costs ten bucks US in onesies.
https://www.digikey.com/product-detail/en/united-chemi-con/ESMH160VSN104MA80T/565-2624-ND/757837

Yrs,
d

On Wed, Aug 2, 2017 at 1:32 PM, Neil N <musomuso@gmail.com> wrote:

> Thats great! So about 5 seconds eh? I'd think that for a festoon the
> cap(s) would have to be much bigger. But maybe 3 of those small LED's
> would be plenty?
>