LED not coming on, or not coming on to full brightness (and LED wiring in general)

For Christmas I received a set of LED bed lights

$103 RealTruck TruXedo Truck Luggage B-Light Tonneau Lighting System | 1704523 | Under rail truck bed lighting system

similar to these. I wired them in through a spring-loaded, ground-interrupt, pin-switch with power coming directly from the battery (a temporary measure for the sake of convenience until I get a secondary fuse panel installed). The two sides are wired in a parallel, and there is a verified, good chassis ground.

For the first month or so, they worked perfectly, lovely and bright, and then randomly they wouldn't come on right away, and/or at less than full brilliance. Now, they either don't come on at all, or come on so dimly as to be almost invisible.

I've noticed this in other inexpensive LEDs as well (non-automotive), so..

1. Is this a wiring issue, or a cheap (and now faulty) LED issue?

2. Are there any fixes for this (unlikely)?

3. I'm looking for recommendations on good quality LED bed lighting (preferably something that tucks up nicely under the lip of the bed rail.