If you are using Magento, this free plugin will auto enable GTM with the dataLayer values. The GTM-account ID is set within the configuration.

https://github.com/CVM/Magento_GoogleTagManager

http://www.magentocommerce.com/magento-connect/google-tag-manager-3.html

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Note: a separate GTM-xxxx container id can be set within Magento’s store view level for dev.clientdomain.com and www.clientdomain.com see details here. This plugin does not yet support enhanced ecommerce or onclick one-page-checkout tracking here, and a JSmacro patch is needed for previousVisitorLifetimeValue calculation here.

Also Fooman has added GTM dataLayer support, but it`s still in beta here & W3C magento plugin is here.

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