Systemd – Writing a Service to Execute at Resume

my Dell laptop is subject to this bug with kernel 3.14. As a workaround I wrote a simple script

/usr/bin/brightness-fix:

#!/bin/bash

echo 0 > /sys/class/backlight/intel_backlight/brightnes

(and made executable: chmod +x /usr/bin/brightness-fix)

and a systemd service calling it that is executed at startup:

/etc/systemd/system/brightness-fix.service

[Unit]
Description=Fixes intel backlight control with Kernel 3.14

[Service]
Type=forking
ExecStart=/usr/bin/brightness-fix
TimeoutSec=0
StandardOutput=syslog
#RemainAfterExit=yes
#SysVStartPriority=99

[Install]
WantedBy=multi-user.target

and enabled: systemctl enable /etc/systemd/system/brightness-fix.service

That works like a charm and I can control my display brightness as wanted.
The problem comes when the laptop resumes after going to sleep mode (e.g. when closing the laptop lip): brightness control doesn't work anymore unless I manually execute my fisrt script above: /usr/bin/brightness-fix

How can I create another systemd service like mine above to be executed at resume time?

EDIT:
According to comments below I have modified my brightness-fix.service like this:

[Unit]
Description=Fixes intel backlight control with Kernel 3.14

[Service]
Type=oneshot
ExecStart=/usr/local/bin/brightness-fix
TimeoutSec=0
StandardOutput=syslog

[Install]
WantedBy=multi-user.target sleep.target

also I have added echo "$1 $2" > /home/luca/br.log to my script to check whether it is actually executed.
The script it is actually executed also at resume (post suspend) but it has no effect (backlit is 100% and cannot be changed). I also tried logging $DISPLAY and $USER and, at resume time, they are empty. So my guess is that the script is executed too early when waking up from sleep. Any hint?